WMG / Happy Death Day 2U

Per wiki policy, Spoilers Off applies here and all spoilers are unmarked. You Have Been Warned.

The time loops are dependent on proximity to Sisi and time of death.

• In the first movie, the time loop latched onto Tree because of all the people in relatively close proximity to Sisi, she had the least amount of time left to live. Once she prevented her death, the loop transferred to Ryan since it was his turn to die. But when Tree was knocked into the alternate timeline, the loop returned to her. If true, this makes for some serious Fridge Horror since it means that DARPA (and possibly even Tree herself) are repeatedly killing Danielle in the name of a science experiment.

After Tree goes back to her own universe, Alt!Carter starts dating his universe's Tree

Because without losing her mother, Tree didn't become bitter or bitchy. While that wasn't enough to keep Carter from dating Danielle (although we never find out how long; it could be a very new relationship), he's just spent a day with Tree being her best, most awesome self, and he seemed very impressed, even before she more or less told him that 'her' Carter was the reason she was going back, and that rather epic kiss. After that, at the very least he'd want to get to know his universe's version of Tree. Alt!Carter hasn't changed from the person that Tree fell for originally, and as long as Alt!Tree's close to the person that Tree became after her Character Development (and why wouldn't she be?) it seems like it would be a sure thing that they'd fall for each other all over again.

As the first movie plot-referenced Groundhog Day and the second movie plot-referenced Back to the Future Part II, the third will plot-reference The Terminator.

• The first movie introduced the time loop from Groundhog Day, the second introduced parallel universes from Back to the future II. What other movie franchise is famed for time shenanigans? It would also follow the natural path of Tree's emotional progression: the first movie had her coming to terms with the present (she realized she wasn't a good person, and resolved to change for the better), the second movie was about her reconciling with her past (her mother's death). What else is there to do but safeguard her future?

Given the first two movies were set in 2017, the third would start in 2022 or so (so that no one would look much older). Ryan and the boys from DARPA finally perfect Sisi, but things go wrong (of course) at the grand unveiling ^note  after all, Ryan never did figure out why Sisi started the time loop in the first place , and it throws Tree back to 2017 again. Now, not only does she have to close the time loop, but do so without giving Carter and Ryan too many hints as to their futures, so they don't accidentally change the course of history so much Tree won't recognize the future she goes back to.

• It could even be resolved by Tree having to go back to September 17, in order to interfere with Sisi and start the original time loop in the first place.

In #3, Carter is the one in the time loop, becoming a true Deuteragonist

• Ryan showed at the start of 2U that Tree wasn't the only person who could time-loop. While the Stinger seemed to set up Danielle as the looper for number 3 during DARPA's experiments... um, who the hell would knowingly pick a spoiled, bitchy, Brainless Beauty sorority chick as the centerpiece for a highly confidential, very strange experiment that could have permanent effects on the body and mind? Actual, controlled experiments have properly prepared and screened volunteers for that sort of thing, even under-the-table Government experiments. ^note not to mention, the audience would have to suffer through HOW many loops before Danielle figures it out, or spends enough time with Tree for her to figure it out? Given Tree's expression as she tells the DARPA agent she knows the perfect person for their experiment, Danielle sitting up in bed and screaming could just be Tree's Imagine Spot.

Besides, Jessica Rothe's winning performance as Tree was a HUGE part of the first two movies. On the other hand, Tree's emotional journey has finished: she's worked through the issues of her past and determined to become a better person in her present. Carter, on the other hand, is not only Ryan's roommate (giving him a good reason to be present for any issues with Sisi), but already knows the basics of time-looping from Tree. This will give him a chance to see exactly what she's gone through, including the growth of their relationship from only one side, and make sure that Rothe is still front and center in the narrative.

• this could even lead to the choice for the Babyface Killer: an Alt!Carter. 2U had a murderous Alt!Ryan trying to kill the original Ryan while he was time-looping, who disappeared (presumably sent back to his own universe once Sisi was fixed). If Sisi malfunctions and sends Carter through a time-loop, what about an Alt!Carter doing the same thing for a different reason: Alt!Carter lost Tree (possibly to another killer version of Lori), and now he's in a universe where she's alive, he'll do whatever he has to in order to stay with her, which means getting rid of Original!Carter. This would even tie into Tree's dilemma from 2U.)

In #3, the Babyface Killer will be Stephanie Butler

• In #1, Lori killed Tree over her affair with Gregory Butler. In 2U, Gregory Butler was trying to kill Lori to cover up their affair, and Stephanie was helping him in order to avoid scandal. In the alternate ending for #1, after she fixes the time loop, Tree was murdered by Stephanie as revenge for sleeping with her husband. Following this progression, #3 might have Stephanie finding out about Tree's (now-ended) affair with Gregory because of the scandal of Lori's attempted murder and subsequent death at the end of #1, and deciding to kill Tree in revenge.
  • Wouldn't that be a little repetitive, seeing that she is the other killer in this timeline?

In #3, Dr Gregory Butler will be an ally to Tree and her friends

• Much like how Lori gained Adaptational Heroism in #2.

SISI was triggered by another SISI in a different timeline

It's mentioned that SISI fired on its on at 12:01 A.M. September 18, 2017. The group have no idea why it fired by itself. Since it's established that there's more than one timeline, what if Tree getting caught in the loop isn't because of death, but because another version of Tree was working on SISI and got caught in SISI's field during an all-nighter?

Or, in a different timeline, Lori went after a Tree on the night of the 17th, chased her to the Quantum Physics lab where an experiment was in progress, a fight ensued and it triggered SISI; maybe the killer even threw Tree into SISI. Its firing "cross-connected" with other SISI reactors in other dimensions and caused their own time loops. Since Tree was in close physical proximity to SISI, it caught the other versions of Tree in their own loops.

It's possible that the same thing happened with Ryan. Or perhaps even with everyone else in the room. Dean Bronson had a rather awed expression on his face afterward.

For the 3rd movie, the looper and/or the killer will keep changing each loop.

This is based on comments that the third movie will tie in elements of Freaky, which involves body-switching. This could go a few different ways:

1. Who the killer is keeps switching each time Tree loops, which makes it a lot more difficult to pin them down and stop them. It might even be a case that magic Aztec artifact from that other movie is ultimately the true Big Bad, meaning she has to find a way to get it to posses none of her friends before she can finally win (maybe stick it with Tombs if he hasn't been executed yet?)
2. The looper keeps changing each time. So first time around from Loop 1 to 2 it might be Tree, then it might be Carter from 2 to 3, then Ryan from 3 to 4, etc. Tree would have to be around to put the idea of looping in everyone's heads for it to persist, even if she isn't the one actually looping since she'd at least have familiarity with the concept. The obvious investigation choice is to see who never loops since having the killer be a looper might be too OP. Unless the killing is caused by some outside malevolent force acting through one of the characters and it becomes something to try to fight back against internally. Like, Iunno, Tree realizing it's trying to harness all her anger and fury at being stuck in so many time loops and she has to do Fighting from the Inside to stop herself.
3. Both are happening, which makes it a complicated game of cat and mouse.

Lori will show again in the 3rd movie, but a Lori from another dimension.

She'll initially serve as the prime suspect by Tree because Lori was the killer in her original story, but like in 2U she'll be exonerated rather early. (Unless a "killer-is-different-each-loop" scenario is used.)

• Or she'll be the Lori from the 2nd movie and their last interaction sets it up for her and Tree to meet again; this time with Lori fully aware of the situation and working side-by-side with Tree.

Another Tree from another dimension is the one responsible for starting the time loops and is the antagonist

In another dimension, there's another Tree that also lost her mother but couldn't let go and move on. Ether she stumbles across Ryan's thesis project or is a participant who becomes obsessed with time travel to save her mother. Unlike the original Tree, this Tree is even worse—broken to the point of being like original Lori—and willing to do anything to get her mother back, including theft and murder. She was the one responsible for setting off her dimension's SISI at 12:01 on the 18th, but ends up in a time loop like the others. After getting out of her loop, she doesn't change for the better but instead more determined and obsessed. This eventually leads to a showdown between her and the original and alternate Trees.

The events of this film lead to the creation of two badass time travel/dimension-hopping crews lead by Tree

It's made clear that Tree has bonded with Ryan, Dre and Samar over the course of the film, and her parting words to Ryan clearly leave a lasting impact on him. The same with her last conversation with Lori. After Lori recovers and talks to the returned Tree and Carter, she finds out the truth and joins them as The Medic. Likewise, just as in Tree's "home" dimension, this Lori and Tree accompanies the group to DARPA and provides critical information that leads to the first practical time machine and inter-dimensional travel. This eventually leads to the two groups meeting and uniting against a common threat.

The other timeline's Tree likely performed community service for prime Tree's actions in the final loop.

Other Tree performing community service instead of getting expelled and charged with a felony for stealing an officer's sidearm and pointing it at said officer is partly due to the officer's incompetence. No one was guarding Tombs at the time he was freed. Even under sedation, there should've been someone guarding the prisoner at all times. He also wasn't entirely listening to Tree when she warned him about the situation and told him to get help. When the authorities found Tombs dressed and armed with a knife and the other killer dressed the same way, Tree would've had a very strong case for mitigation. Also, any inconsistencies in her statement could be dismissed as the effects of a concussion, which other Lori witnessed during Tree's fight with Gregory.